Nd-Fe-B permanent magnet alloys have high residual flux density (Br) and high intrinsic coercivity (iHc), so that they have been receiving much attention as new permanent magnet materials which supplant conventional permanent magnets such as alnico magnets, hard ferrite magnets and Sm-Co magnets See Japanese Patent Laid-Open Nos. 59-46008, 59-64733 and 59-89401, and M. Sagawa et al., "New Material for Permanent Magnets on a Base of Nd and Fe," J. Appl. Phys. 55 (6) 2083 (1984). These Nd-Fe-B magnets consist essentially of 8-30 at % of Nd and/or Pr, 2-28 at % of B and balance Fe. They may contain additional elements such as Co., Al, Dy, Nb, Ti and Mo (Japanese Patent Laid-Open No. 59-219453).
Such permanent magnets may be prepared by powder metallurgy. Specifically speaking, component elements in a proper proportion are mixed and melted to form an ingot which is then pulverized and milled. The milled material is sintered and then heat-treated.
The heat treatment conditions may vary depending on the types of rare earth elements and the composition of magnets, but the Nd-Fe-B sintered magnets are usually annealed at temperatures of around 600.degree. C. For instance, Sagawa et al. reported that the annealing at 590.degree.-650.degree. C. provides R-Fe-B magnets with intrinsic coercivity (iHc) of as high as almost 12 KOe. See J. Appl. Phys. 55 (6), 2086 (1984). These R-Fe-B alloys have (BH)max of up to about 35 MGOe, which is much higher than (BH)max of R-Co magnets which is at most about 30 MGOe.
However, Nd-Fe-B permanent magnets subjected to the conventional heat treatment have intrinsic coercivity (iHc) which varies widely depending on the composition, grain size, oxygen content and sintering temperature. In other words, the conventional heat treatment fails to draw sufficiently a potential iHc which such magnet materials inherently have.